The Tin Man's Real Name Was Nick Chopper
L. Frank Baum's twelfth book, The Tin Woodman of Oz, tells us all about the Tin Man, maybe more than we wanted to know. It turns out his real name is Nick Chopper, which is quite a coincidence, if you think about it. After falling in love with the Wicked Witch of the East's ward, Nimmie Amee, the witch enchants his axe, forcing it to cut off all his body parts one by one. A tinsmith named Ku-Klip replaces each dismembered body part with a metal version. A heavy metal version.
It actually gets even crazier from there, as the Tinman comes across his ex-lady's next lover, Captain Fy-ter, who was also forced to dismember himself via the evil witch's magic. The two then vie for Nimmie Amee's affections in a truly bizarre love triangle. Sparks fly, but that's not a good thing.
The Wizard of Oz Had a Mean Streak
In the first Oz book, The Wonderful Wizard of Oz, the wizard is much as we know him today: a kind of cowardly liar who means well. In the sequel, The Marvelous Land of Oz, however, we learn that the wizard helped the evil witch Mombi usurp Oz's rightful ruler, King Pastoria II, and kidnapped his daughter Ozma, turning her into a boy. This was all ignored in later books. In fact, Glinda eventually gives the Wizard powers so he can be an actual wizard instead of just a humbug.
Oz Once Had a Transgender Ruler
Reilly & Britton Publishing
As hinted at in the entry above, The Marvelous Land of Oz is all about a boy named Tip who discovers he is actually a girl who had been transformed into a boy with magic. This comes as quite a shock to him, as you can probably imagine. But everyone gives him a pep talk, and soon he accepts the loss of his wiener. As Ozma, she ends up ruling Oz quite well and repels several attempts to steal her throne.
Dorothy and Auntie Em Eventually Live in Oz Forever
Despite all her adventures to Oz, Dorothy can never convince Aunt Em or her lesser known Uncle Henry that she really does hang out in a magical land with witches and wizards. But when debt from the destruction of their farmhouse by The Wonderful Wizard of Oz's tornado gets to be too much, Dorothy has her family transported to Oz to live out the rest of their days, as well as to give the world's biggest "I told you so."
The Land of Oz is Actually on Earth Somewhere
Oz is not it's own planet, nor is it some dream world you visit after hitting your head especially hard. Rather, it seems to exist somewhere on our planet. Geographically, it's a rectangular shaped region consisting of four parts: Munchkin Country, Winkie Country, Gillikin Country, and Quadling Country. Each country is color-coded. For instance, everything from Gillikins is purple, even the grass and mud, though this is not consistent from book to book.
The land is surrounded by a magical desert which keeps people safe from invasion. And beyond that there are other lands such as the Kingdom if IX and the Land of Ev. Oz and these lands exist on a continent known as Nonestica which rests in the Nonestica Ocean. So if you just figure out where that is, you too can visit Oz.
The Hungry Tiger Was Originally Part of the Gang
Hungry Tiger is a tiger who hangs out with the Cowardly Lion and has a tummy that can never be filled. The poor guy longs to eat a fat baby, but never does because he would feel too bad afterward. When he meets new people, he asks for their permission to eat them. When they say no, he gets bummed out. He is gigantic, and probably not very smart since he thinks a dentist might be able to pull out his appetite along with this teeth. Oh, how I wish The Wizard of Oz featured this guy instead of the Cowardly Lion.
The Munchkins Weren't Actually Short
We know the Munchkins as shorter than average people because they were all played by adult proportional dwarves in The Wizard of Oz. But in the books, most adults are apparently as small as Munchkins with the exception of Glinda. Baum describes the citizens of The Emerald City as the same size as Munchkins and describes the Quadlings (citizens of Quadling Country, of course) as short, fat people. If you look at W.W. Denslow's Baum-approved illustrations for The Wonderful Wizard of Oz, the Scarecrow, the Tin Man, and even the Wizard himself are all just about as tall as little Dorothy.
There are Lots and Lots of Witches
There are tons of witches in Oz lore, not the two and a half we meet in the first film. Along with Glinda the Good Witch (who controls the South), there is the Good Witch of the North. We also have the really scary Wicked Witch of the East, who Dorothy accidentally kills with her house, and the Wicked Witch of the West, who we all know from the film, though in the book she has only one eye, constantly carries an umbrella, and controls a lot more animals than just a couple measly flying monkeys.
These witches do not have equal power levels. For instance the Good Witch of the North got her position by throwing out former Evil Witch of the North, Mombi, but she has no power that can stand up to the cruel Wicked Witch of the East, otherwise she would have liberated the Munchkins from her rule. And Glinda is just an all round badass.
There Are Over 50 'Oz' Books
L. Frank Baum wrote fourteen main Oz novels before his death. After he died, however, publisher Reilly & Lee kept pumping Oz books out. Most (19 of them) were written by Ruth Plumly Thompson, who utilized much more traditional fairy tale tropes than Baum. Nevertheless, it can be hard to tell just by looking at their covers and titles which ones are his and which ones are hers. So, as with everything in life, always make sure you check the byline.
The Main 'Oz' Villain Isn't the Wicked Witch
The main antagonist working against Oz is not the Wicked Witch of the West, who Dorothy takes out in the very first novel, but the Nome King. (That is not a misspelling, by the way. Until you get to the Ruth Plumly Thompson books, it's "Nome" not "Gnome.")
The Nome King (aka Roquat the Red or sometimes Ruggedo) really wants to take over Oz, his former kingdom. He lives underground and resents all surface dwellers, often keeping them as slaves just to be spiteful. Through magic and other sneaky methods he's able to front several invasive attempts to take back his land, but none work. He is terrified of eggs.